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Analytical Chemistry

Enantioselectivity With Microwaves

This new paper is a very interesting approach to chiral separation, and I would like to go into detail about how it works. Unfortunately, section 2 of the Supplementary material goes into detail, and it’s titled “Three-level optical Bloch equations”, and I can just about follow it until I get to the part that says “The time evolution of the density matrix is then expressed by the Liouville equation“, and equally unfortunately, that’s at the end of the first paragraph. So someone with more physics than I have in my pockets will have to comment on the mechanism.

But the upshot is that they’re separating enantiomers by pulsing various polarized microwave bursts at them – you can promote either the R or S to a higher rotational state selectively. This sort of thing in general (enantioselectivity through electromagnetic means) has a long and not particularly successful history. It ranges from extremely tiny (but real) effects that are sometimes brought into origin-of-life debates (why did life on Earth settle on the enantiomeric series it did?), all the way to large, attention-getting, and completely unreproducible claims such as those from a group at Bonn in the 1990s, who published on enantioselective reactions in high magnetic fields until the work turned out to have been faked up by one of the coauthors (as I recall the story).

This work is certainly far more firmly grounded, and is a follow-up to the research program described in this review. Here’s the abstract:

We report the experimental demonstration of coherent enantiomer-selective enrichment of chiral molecules by employing a novel microwave five-pulse scheme. Our results show that enantiomers can be selectively transferred to a rotational level of choice by applying sequences of resonant microwave pulses in a phase- and polarization-controlled manner.

Unfortunately, I don’t have access to the full text, so I’m going to have to add more when I do. I did want to get this out there for comment, though, because it would seem to have implications for analytical chemistry (and perhaps possibly synthetic methods?) Look to the comments here for more from people who have better library access than I do at the moment, and better grasps of rotational spectroscopy as well. . .

24 comments on “Enantioselectivity With Microwaves”

The rotational energy levels (and therefore the gaps between them) are the same for both enantiomers. Unless you contrive to orient the molecules (in a crystal?) I’d be astonished if one enantiomer could absorb a (polarize) photon better than its enantiomer. But even in the gas phase, kinetic energy (however it was pumped in) is rapidly shared by collisions with other molecules

That’s one thing that puzzled me, but I figured that if d and l enantiomers can polarize light in different directions maybe they can absorb polarized light differently too. So why hasn’t someone done this with polarized light, which seems more likely to work than polarized microwaves?

Which brings up the other question I have. What is the microwave chromophore (or whatever you call it) that absorbs the microwaves? Are these molecules in solution? I suppose the microwaves could be interacting with the solvation shells which would of course themselves be chiral, but that seems like a far-fetched possibility.

“That’s one thing that puzzled me, but I figured that if d and l enantiomers can polarize light in different directions maybe they can absorb polarized light differently too. So why hasn’t someone done this with polarized light, which seems more likely to work than polarized microwaves?”

This has actually been done with circular polarized light, and was shown to work quite well. The diffrent absorbtion of circular polarized light is the principle after which a common CD-spectrometer works. You can also use this for enantioenrichment, as demonstrated by Kuhn as early as the 1930s. The only problem is that the ees are very small, for theoretical reasons, that can not be easily circumvented.

No. Obviously there is an interaction of the linearly polarized light with the chiral molecules, but it isn’t an absorption process (by which I mean the photon energy of the polarized light doesn’t have to be resonant with a transition).

I meant to say “rotate”, not “polarize”, but I had polarization on the brain. I considered posting a correction a few seconds later, but I dislike it when other people post minor corrections like spelling errors, so I decided to let it ride. In retrospect, a poor decision.

One of the great accomplishments of science has been to demonstrate the underlying unity
among various superficially different phenomena. One of the aims of teaching science should he to point out this unity whenever possible. The purpose of this paper is to indicate a neglected opportunity to do just this in elementary physical chemistry. The topics to
be discussed are light absorption, dispersion, circular dichroism, and rotatory dispersion. These very closely related phenomena are all of importance in chemistry and yet, to the writer’s knowledge, no elementary physical chemistry texts discuss them as a unified topic.
This is especially unfortunate since on a phenomenological level it could he very easily done.

This is odd but I am actually sitting listening to Kenso Soai as I write talk about circularly polarised enrichment using autocatalysis. You can can get ees up to 99% using CPL as the small excess produced at the start gets magnified by the reaction using autocatalysis

Get your enantiomerically enriched water now! Using specially polarized microwaves, we are able to turn all that bad acidic left rotating water into perfectly hexagonally structured alkaline right rotating water.
We are currently working on using this technology to imprint the rotational fingerprint of medical substances onto the water in order to use it to cure cancer.

I’m afraid I’m not sufficiently expert in microwave spectroscopy to really pass judgement on this (although I’ve sat through plenty of rotational spectroscopy talks with the obligatory huge tables of rotational constants). Still, this is a very long way from something which could be routinely used for analysis.

In terms of actual physical separation or purification, another step is going to be required to separate the two enantiomers in different rotational states – I suspect this is not as straight forward as might be implied in the closing paragraph of the paper.

the CPL induced enantiodestruction to make enaptioenriched molecules is well know, Kagan showed you can get respectible levels of ee at high conversion using a few different substrates in the 70s. Since then this has been coupled to amplification processes (Soai as mentioned above) in liquid crystals and supramolecular species (Fearing mostly), the formation of helicenes, and explored in prebiotic scenarios. I wrote a highlight of some nice work by the Kellogg / Vlieg group here http://www.nature.com/nchem/journal/v1/n9/full/nchem.455.html